1. Field of the Invention
This invention is directed to branched alkylpolyethoxypropane sulfonates and a process for their use in enhancing the secondary or tertiary recovery of oil from subterranean oil deposits or reservoirs, particularly from high salinity reservoirs. In particular, branched alkylpolyethoxypropane sulfonates are suitable as single component surfactants in continuous chemical flooding techniques. They are effective over a broad range of salinity and at extremely high dilution.
II. Discussion of the Prior Art
In the recovery of oil from oil bearing deposits it is generally possible to recover only a portion of the original oil by so called "primary methods" which utilize only the natural forces present in the reservoir or deposit. Thus a variety of supplemental techniques have been employed in order to increase the recovery of oil from these subterranean reservoirs. The most widely used supplemental recovery technique is water flooding which involves injection of water into an oil bearing reservoir. However, there are problems associated with the water flooding technique and water soluble surfactants have generally been required to be used for this process to be completely successful. Thus the LTWF (Low Tension Water Flood) method using surfactants which function in low salinity (less than 3 percent) is well known. However, it has been found that preflushing the reservoirs with fresh or low salinity water to reduce the salinity so that the low salinity surfactants of the prior art may be used is not always effective, or, the preflushing is effective only for a short duration and the salinity of the fresh water increases over a period of time since it is in contact with reservoir rocks and clays. Either event renders the low salinity surfactants useless and therefore it is of vital importance to have a surfactant which functions at the salinity of the connant water to negate the necessity of preflushing.
Essentially two different concepts have developed for using surfactants to enhance oil recovery. In the first, a solution containing a low concentration of surfactants is injected into the reservoir. The surfactant is dissolved in either water or oil and is in equilibrium with aggregates of the surfactant known as micelles. Large pore volumes (about 15-60% or more) of the solution are injected into the reservoir to reduce interfacial tension between oil and water and thereby increase oil recovery. Specific relationships exist between interfacial tensions of the oil against the flooding media and the percentage recovery obtained by flooding, i.e., the efficiency of flooding increases as the interfacial tension decreases. Oil may be banked with the surfactant solution process but residual oil saturation at a given position in the reservoir will only approach zero after passage of large volumes of surfactant solution.
In the second process, a relatively small pore volume (about 3-20%) of a higher concentration surfactant solution is injected into the reservoir. With the higher surfactant concentration, the micelles become a surfactant-stabilized dispersion of either water and hydrocarbon or hydrocarbon and water. The high surfactant concentrations allow the amount of dispersed phase in the microemulsion to be high in comparison to the dispersed phase of the micelles in the low concentration surfactant solutions.
The injected solution (slug) is formulated with three or more components. The basic components (hydrocarbon, surfactant, and water) are sufficient to form the micellar solutions. A cosurfactant fourth component (usually alcohol) can be added. Electrolytes, normally inorganic salts form a fifth component that may be used in preparing the micellar solutions of microemulsions. The high concentration surfactant solutions displace both oil and water and readily displace all the oil contacted in the reservoir. As the high concentration slug moves in the reservoir, it is diluted by formation flood and the process reverts to a low concentration flood; Enhanced Oil Recovery, Van Poolen & Associates, 1980, Tulsa, Okla.
Work is still in progress in the laboratory and in the field to select the optimum method of injecting surfactant to improve oil recovery. The best process for a specific reservoir is the one which has the potential to provide the greatest efficiency and yield regardless of the concentration level of the surfactant. The chemical system, however, to be efficient must be tailored to the specific reservoir.
The prior art with respect to the use surfactant polymer floods to recover oil from reservoirs has disclosed that for a given amount of surfactant, a small slug process with a high surfactant concentration is more efficient than a large slug process with a low surfactant concentration. The former produces oil earlier but takes a smaller number of pore volumes to complete oil production. This is a favorable condition. However, it has become evident that fluid dispersion and mixing take place in the reservoirs and the slug intake routine cannot be maintained. Deterioration of the surfactant and the mobility control slug can lead to process failure or at least a reduction in process efficiency. For heterogeneous reservoirs where fluid dispersion and mixing takes place to a greater extent it is desirable if not vital to have a continuous flooding process with a surfactant which can move oil even at very low concentrations.
We have now discovered certain novel surfactants and their use in a continuous flooding process wherein low concentrations of the novel surfactant alone can be used to increase the oil production during secondary water flooding processes or to recover residual tertiary oil where the reservoirs already have been water flooded.